Over 90 percent of all marine species and 70 percent of land vertebrates perished as a result, according to the NASA-funded research team, led by Dr. Luann Becker of the University of Washington (UW), Seattle. The team's findings will be published tomorrow in the journal Science.

The collision wasn't directly responsible for the extinction but rather triggered a series of events, such as massive volcanism, and changes in ocean oxygen, sea level and climate. That in turn led to species extinction on a wholesale level, according to the team.

"If the species cannot adjust, they perish. It's a survival- of-the-fittest sort of thing," said Becker, UW acting assistant professor of Earth and Space Sciences. "To knock out 90 percent of organisms, you've got to attack them on more than one front."

The scientists do not know the site of the impact 250 million years ago, when all Earth's land formed a supercontinent called Pangea. However, the space body left a calling card -- complex carbon molecules called buckminsterfullerenes, or Buckyballs, with the noble gases helium and argon trapped inside the caged structure. Fullerenes, which contain at least 60 carbon atoms and have a structure resembling a soccer ball or a geodesic dome, are named for Buckminster Fuller, inventor of the geodesic dome.

The researchers know these particular Buckyballs are extraterrestrial because the noble gases trapped inside have an unusual ratio of isotopes, atoms whose nuclei have the same number of protons but different numbers of neutrons. Terrestrial helium is mostly helium-4, while extraterrestrial helium is mostly helium-3.

"These things form in carbon stars. That's what's exciting about finding fullerenes as a tracer," Becker said. The extreme temperatures and gas pressures in carbon stars are perhaps the only way extraterrestrial noble gases could be forced inside a fullerene, she said.

These gas-laden fullerenes were formed outside the Solar System, and their concentration in the sedimentary layer at the boundary of the Permian and Triassic periods means they were delivered by comets or asteroids. The researchers estimate the comet or asteroid was roughly 3 3/4 to 7 1/2 miles (6 to 12 kilometers) across, or about the same size as the asteroid believed responsible for the extinction of the dinosaurs 65 million years ago.

The telltale fullerenes containing helium and argon were extracted from sites where the Permian-Triassic boundary layer had been exposed in Japan, China and Hungary. The evidence was not as strong from the Hungary site, but the China and Japan samples bear strong evidence, Becker said.

The team's work was made more difficult because there are few 250 million-year-old rocks left on Earth since most rocks of that age have been recycled through the planet's tectonic processes. "It took us two years to do this research, to try to narrow it down enough so that we could see this fullerene signature," Becker said.

Scientists have long known of the mass extinction 250 million years ago, since many fossils below the boundary -- such as trilobites, which once numbered more than 15,000 species -- diminish sharply close to the boundary and are not found above it. There also is strong evidence suggesting the extinction happened very rapidly, on the order of 8,000 to 100,000 years, which the latest research supports.

Previously, it was thought that any asteroid or comet collision would leave strong evidence of the element iridium, the signal found in the sedimentary layer from the time of the dinosaur extinction. Iridium was found at the Permian-Triassic boundary, but not nearly in the concentration as from the dinosaur extinction. Becker believes that difference is because the two space bodies that slammed into Earth had different compositions.

Members of the research team are Becker; Robert Poreda and Andrew Hunt from the University of Rochester, NY; Ted Bunch of the NASA's Ames Research Center, Moffett Field, CA; and Michael Rampino of New York University and NASA's Goddard Institute of Space Sciences, New York. Funding for the research was provided by NASA's Astrobiology and Cosmochemistry programs and the National Science Foundation.

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